on a mac air notebook I have no trouble opening other tgz downloads from GPS file depot. Today I downloaded and opened North and South Dakota maps in that file format. However,MT Topo + National Forests 2016.gmapi-87.tgz ....... I have downloaded it three times. I have changed extension to .zip. I have tried using UNIX commands in terminal mode. None of the should be necessary. I suspect the file is corrupted.
I assume this map? https://www.gpsfiledepot.com/maps/view/87/downloadmac/
I was going to try it on my Mac and started the download, but it's a 600MB file and I'm in a rural location with a very slow DSL connection. Sorry, the estimates were saying an hour and that's just too much.
But from what you describe, it does sound like a corrupt file. I don't know about the author of that map, but many of the other authors here don't have Macs and therefore haven't personally checked their maps.
OTOH, there are almost 3,500 downloads of that map and you'd think there would have been other complaints if it didn't work. You can send the author a PM here: https://forums.gpsfiledepot.com/index.php?action=profile;u=176
I opened it in winrar on Windows (no mac). I'll convert it to a .zip and am uploading a new version for you now. I'll edit this post once its uploaded.
EDIT: The new .zip file is up; try it.